1. Field of the Invention
The present invention relates to a technology for increasing a breakdown voltage and reliability of a semiconductor power device, and more particularly to a packaging technology for preventing a dielectric breakdown of a power device module which has a plural of power switching devices mounted thereon and for increasing a breakdown voltage and reliability thereof.
2. Description of the Related Art
Semiconductor power devices for converting and controlling the flow of electric energy have been greatly researched and developed in recent years in response to high requirements from various fields including the power electronics field at first. It is particularly required to realize a power switching device, for example, a Power MOSFET, a Power Bipolar Transistor (Power BJT), a Gate Turn-off (GTO) Thyristor and an Insulated Gate Bipolar Transistor (IGBT) that can control a high power and achieve a high performance. As the result, a high voltage semiconductor power device called an Intelligent Power Module (IPM) is proposed and developed, which integrates a plural of power switching devices and a semiconductor chip that includes a circuit for controlling these devices in a single package. The IPM is currently applied to an inverter such as a direct drive circuit of an energy reduction type and an intelligent actuator, and is also expected to apply to other fields. In addition, it is particularly required to make the IPM have a much higher breakdown voltage.
To increase the breakdown voltage of such the semiconductor power device, it can be considered currently to improve a packaging technology that employs a substrate with a direct bond structure capable of achieving an advanced planning for heat radiation. The packaging technology that employs the substrate with the direct bond structure means such a packaging technology that employs a direct bond copper (DBC) substrate, which consists of a high heat conductive aluminum nitride (hereinafter, referred to as xe2x80x9can AlNxe2x80x9d) substrate and a copper film attached on the surfaces of the AlN substrate, and that can reduce a heat resistance and simplify the structure.
FIG. 1 is a cross sectional view showing a configuration of a package for a semiconductor power device with the DBC substrate. In the package, a plural of semiconductor chips 21, 22 and 23 are disposed on a copper film 1a that is attached on a top surface of an insulating AlN substrate 2 and another copper film 1b that is attached at a bottom surface of an insulating AlN substrate 2 except a peripheral region thereof. The semiconductor chips 21, 22 and 23 are connected electrically with each other via lead wires 31 and 32 such as bonding wires and ribbon wires. These semiconductor chips 21, 22 and 23 may, for example, include semiconductor power chips 21, 23 and a control circuitry chip 22 for controlling the semiconductor power chips 21 and 23.
The copper film 1b bonded to the bottom surface of the AlN substrate 2 is attached to the center of a metallic heat sink 5. A case (body of a container) 6 is equipped on a peripheral region of the heat sink 5 to surround the entire AlN substrate 2 as shown in FIG. 1.
A terminal holder 8 has external terminals (electrode terminals for external connection) 71, 72 and an aperture 8a, and is fixed at an upper portion inside the case (container body) 6 in order to close the outer case 6. The external terminals 71 and 72 are employed such members as to realize electrical conduction to external from each of the semiconductor power chips 21, 23 and the control circuitry chip 22 inside the case (container body) 6.
A silicone gel 91 or an insulating material is flowed through the aperture 8a into a module which is surrounded by the conductive bottom plate 5 used as the heat sink, the case (container body) 6 and the terminal holder 8. An epoxy resin is thereafter provided on the silicone gel to seal. After sealing with the epoxy resin, arranging the terminal holder 8 to close the case, which may consist of a material as same as or different from that of the outer case, then closing the aperture 8a with sealing materials 81 and 82 after setting the silicone gel 91.
In the particular case where the IPM is used in a high voltage power equipment, a hygroscopic property presented in the silicone gel 91 may degrade electric characteristics. Therefore, suitable adhering methods and adhering structures, which can suppress generation of openings and joints among the components by strong bonds between the outer case 6 and the heat sink (the conductive bottom plate) and between the lid (terminal holder) 8 and the epoxy resin, for example, are employed to prevent humidity from penetrating into the IPM as far as possible. An inner structure such as a space and a pressure-releasing valve is also provided inside the IPM to prevent the silicone gel 91 from swelling and exuding, and to reduce the heat stress applied to the epoxy resin.
The package structure shown in FIG. 1 may improve, by employing a high heat conductive AlN substrate, a thermal conductivity between the conductive bottom plate serving as the heat sink and the substrate, and may also reduce the amount of a molybdenum (Mo) plate and a solder material by bonding the semiconductor chip directly to the AlN substrate 2 that has the top copper film 1a. This is an advanced technology in the radiation planning which can reduce the heat resistance and simplify the structure as mentioned above.
However, the above-described conventional IPM has several technical problems that are derived from the structure thereof. These technical problems will be described below.
First, in order for undergoing to be used under a high voltage, the IPM adopts the adhering method or the adhering structure that can suppress the generation of the openings and joints between different components by the strong bonds. This configuration may play the role of reducing the moisture penetration. However, it is impossible to remove the generation of the openings and joints completely, leaving a tiny opening or joint within the semiconductor device. In addition, bonding the components strongly may increase the heat stress in a portion where different thermal expansion coefficients exist between components such as the outer case 6 and the heat sink (conductive bottom plate) 5, and may cause a breakage of the outer case 6 and an enlargement of the opening remained. The breakage of the outer case 6 and the enlargement of the opening may invite a moisture absorption by the silicone gel, resulting in a cause to degrade electrical properties and durability thereof.
Second, adding a component such as the pressure-releasing valve for relieving the heat stress inside the IPM may complicate an inner structure of the device and increase a size of the entire device.
Third, since the conventional IPM is produced from several materials such as the outer case 6, the lid (terminal holder) 8, the silicone gel 91 and the epoxy resin, the heat stress may be generated due to the differences among the thermal expansion coefficients of the materials, and a complicated force is applied to each portion inside the device, resulting in the breakage and the degradation of the substrate and electronic components in the device. In general, the steps that are required for assembling the device increases in accordance with the number of the materials used, causing a reduction of reliability of the product and an increase of cost.
Fourth, whereas the package structure of the conventional IPM has the advantage for planning the heat radiation to realize a high breakdown voltage, it leaves a problem with respect to the dielectric breakdown. Namely, the package structure mentioned above employs the silicone gel 91 as the insulating material, but the silicone gel 91 has a property in which a dielectric breakdown voltage is lower than those of general solid insulators. Because a creeping distance between the silicone gel 91 and the AlN substrate (a distance from an edge of the semiconductor power chip 21 or 23 to an edge of the AlN substrate 2) is short, a creeping breakdown at the interface between the both may occur. The adhesion at the interface between the both is insufficient, and a creeping discharge may occur easily.
Fifth, in the conventional semiconductor module, when the insulating substrate that receives the semiconductor chip is located directly on the bottom plate 5 of the metallic case (container body) that serves as the heat sink, the dielectric breakdown at the interface between the insulating substrate and the silicone gel may occur easily, because the creeping distance is short and the metallic bottom plate serves as a back electrode during the creeping discharge, extending the creeping discharge with ease.
Such the creeping breakdown and the creeping discharge do not occur when driven under a rated voltage, but are considered to occur during driven under a higher voltage than the rated voltage. Once the creeping breakdown or discharge occurs, the dielectric breakdown voltage is lowered and the dielectric breakdown may be easily caused.
In view of increasing the breakdown voltage and improving the reliability, therefore, suppressing the creeping breakdown and discharge to prevent the dielectric breakdown is required.
The present invention is made in consideration of the above problems, and has an object to provide a package for a semiconductor power device with a high reliability.
Another object of the present invention is to provide a package for a semiconductor power device capable of preventing the dielectric breakdown, increasing the breakdown voltage and improving the reliability.
A different object of the present invention is to provide a package for a semiconductor power device capable of preventing damages to the module and operation defects in the semiconductor chip caused by the heat stress applied to the semiconductor chip and the containing case (container body) due to the thermal expansion and the thermal shrinkage of a filling agent.
A further object of the present invention is to provide a package for a semiconductor power device capable of providing a sufficient waterproof effect to the semiconductor module.
A further different object of the present invention is to provide a package for a semiconductor power device having an advanced electric property and a high durability.
A further object of the present invention is to provide a method for assembling a package for a semiconductor power device having an advanced electric property and a high durability.
An essential point of a first and a second features of the present invention is to realize a higher breakdown voltage of the package for the semiconductor power device, which is achieved with a configuration that can suppress the creeping breakdown and the creeping discharge at an interface of two kinds of insulating materials or the DBC substrate and the silicone gel.
In order to suppress the creeping breakdown and the creeping discharge at the interface of the two kinds of insulating materials, a system for increasing the breakdown voltage by interposing an insulating resin such as an epoxy resin and a polyester resin which has a dielectric breakdown voltage higher than those of both the insulating materials therebetween (the first feature), and a system for relieving the electric field across the interface between the insulating materials (the first and second features) are considered.
(a) The system for increasing the breakdown voltage according to the first feature may also relax the electric field strength. Moreover, it is particularly preferable to interpose a thicker insulator by heaping or adhering a solid in view of suppressing a penetrative puncture.
(b) The system for relieving the electric field according to the first and second features includes a material type which interposes an insulator having a median permittivity of those of both the insulators therebetween, and an engineering type which eliminates any sharp shapes such as corners and rough surfaces that cause the creeping discharge at the interface between both the insulating materials with ease.
(b-1) The following ways are available as the material type:
a way for coating a resin such as the epoxy and polyester between both the insulating substrates; and a way for further filling into the resin a powdered ceramic such as an aluminum oxide (hereinafter, referred to as xe2x80x9cAl2O3xe2x80x9d) and the AlN.
(b-2) The following ways are considered as the engineering type: a way for contacting the copper film tightly with the AlN substrate by covering all the edges and corners of the film with the resin; and a way for polishing to smooth the surface of the outer edge region of the DBC substrate.
On the basis of the above novel knowledge found experimentally by the present inventors, the first feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; a solidified insulating material disposed on an outer edge region of the conductive film and the peripheral region of the insulating substrate; and an insulating material formed on the semiconductor chip. xe2x80x9cThe solidified insulating materialxe2x80x9d may, for example, a hardened resin.
According to the first feature of the present invention, the solidified insulating material disposed on the outer edge region of the conductive film and the peripheral region of the insulating substrate may contact the outer edge region of the conductive film with the peripheral region of the insulating substrate tightly. Further, the solidified insulating material interposed into an interface between the bothxe2x80x94the conductive film and the peripheral region of the insulating substratexe2x80x94may relax an electric field strength across the interface between the both and make it difficult to cause the creeping discharge. As a result, the prevention of the dielectric breakdown, the realization of a higher breakdown voltage of the semiconductor power device and the improvement of the reliability can be achieved. If a height of the solidified insulating material exceeds a thickness of the semiconductor chip, the thick the solidified insulating material can suppress a penetrative puncture in which the discharge punches through the solidified insulating material from the outer edge region of the conductive film, and can prevent the creeping breakdown completely. Containing the powdered aluminum oxide in the solidified insulating material may further relax the electric field strength and make it difficult to cause the creeping breakdown.
In the first feature of the present invention, it is possible to further comprise a container arranged on the bottom plate, surrounding the insulating substrate; an upper lid arranged at an upper portion of the container; an external terminal supported through the upper lid and connected electrically with the semiconductor chip. Then the insulating material can be filled within the container to cover the semiconductor chip.
The second feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate, having a smooth region of a given surface roughness near a peripheral region thereof; a conductive film formed selectively on the insulating substrate, and a semiconductor chip mounted on the conductive film; and an insulating material formed on the semiconductor chip.
According to the second feature of the present invention, an exposed top surface at the peripheral region of the insulating substrate or a bottom surface opposing to the top surface, that is, a plane mating to the conductive bottom plate serving as the heat radiating plane has the smooth region of the given surface roughness, which may not cause a partial discharge from the surface at the peripheral region of the insulating substrate. Thus, the prevention of the dielectric breakdown, the realization of a higher breakdown voltage of the semiconductor power device and the improvement of the reliability can be achieved.
The xe2x80x9cgiven surface roughnessxe2x80x9d means a certain flatness realized by polishing. For example, a surface roughness smaller than 0.05 xcexcm, which is at least one order smaller than the conventional roughness of the insulating substrate may be preferable. A method for polishing to achieve the xe2x80x9csmooth region having the given surface roughnessxe2x80x9d is based practically on the following experiments performed by the present inventors:
When causing the dielectric breakdown in the simple AlN by means of a ball-plane electrode pair in an insulating liquid or a perfluorocarbon (dielectric constant: 1.86), a breakdown position is located at a portion slightly apart from a contacting portion with the ball;
When causing the dielectric breakdown at the AlN substrate in the perfluorocarbon, a breakdown position is located at the edge of the copper film, and a value of the dielectric breakdown voltage increases by about 10%;
On the other hand, when causing the dielectric breakdown at the AlN substrate in the silicone gel (dielectric constant: about 2.8), a breakdown position is located at a portion apart from the edge of the copper film, and the value of the dielectric breakdown voltage further increases by about 10%;
The dielectric breakdown in the perfluorocarbon left partial discharge scars around the electrodes uniformly, while the dielectric breakdown in the silicone gel left tree-like discharge scars locally; and
When causing the dielectric breakdown in the silicone gel after polishing the surface of the AlN substrate outside the edge of the copper film to the given surface roughness by 3 xcexcm has further increased the value of the dielectric breakdown voltage by about 10% or more.
These results are observed similarly in insulating substrates other than the AlN substrate, for example, an alumina (Al2O3) substrate and a beryllia (BeO2) substrate. The present inventors therefore conclude that the number (area) of the partial discharge scars has a relationship proportional to the value of the dielectric breakdown voltage, and that the smoothness of the surface has a relationship proportional to the value of the dielectric breakdown voltage. As the partial discharge may take place due to the irregularity of the surface, and as the dielectric breakdown may occur due to the partial discharge in the AlN substrate, the upper or bottom surface of the peripheral region of the insulating substrate should be completed so as to have the given surface roughness.
A solidified insulating material, or a hardened resin as similar as that of the first feature may be applied on the smooth region of the insulating substrate. Thus, the prevention of the dielectric breakdown, the realization of the higher breakdown voltage of the semiconductor power device and the improvement of the reliability can be achieved.
Similar to the first feature, in the second feature of the present invention, it is possible to further comprises a container arranged on the bottom plate, surrounding the insulating substrate; an upper lid arranged at an upper portion of the container; an external terminal supported through the upper lid and connected electrically with the semiconductor chip. Then the insulating material can be filled within the container to cover the semiconductor chip.
The third feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink, wherein the bottom plate has a notch arranged in a peripheral region of a surface thereof; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; and an insulating material formed on the semiconductor chip.
According to the third feature of the present invention, a creeping distance for a creeping current that is generated when flowing from the semiconductor chip through the insulating substrate may be substantially elongated, and the creeping breakdown voltage of the semiconductor power device may be increased. The notch is preferably located outwardly apart from a region for mounting the semiconductor chip. Thus, as the insulating substrate below the region for mounting the semiconductor chip can contact tightly at least with the heat sink, the thermal radiation effect of the semiconductor chip can be maintained. If an insulating material with a high thermal conductivity is buried within the notch, the notch serves as an insulator. Therefore, the creeping breakdown voltage of the semiconductor device can be further increased certainly, and the radiation of the heat from the insulating substrate can be performed sufficiently.
Similar to the first and second features, it is possible to further comprises a container arranged on the bottom plate, surrounding the insulating substrate; an upper lid arranged at an upper portion of the container; an external terminal supported through the upper lid and connected electrically with the semiconductor chip. Then the insulating material can be filled within the container to cover the semiconductor chip.
The fourth feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; a container arranged on the bottom plate, surrounding the insulating substrate; an upper lid arranged at an upper portion of the container; an inlet formed at a part of the upper lid; an insulating material injected through the inlet to fill the container; and a waterproof and flexible film located at and to close the inlet.
According to the fourth feature of the present invention, even if the thermal expansion and shrinkage of the insulating material due to the variation of temperature occur, the heat stress can be relaxed because of a deformation of the flexible film which closes the inlet for injecting insulating material. A metallic film may be employed as the film. A multi-layer film which consists of stacked waterproof films and metallic films may also be utilized. A multi-layer structure which has a film in at least one layer capable of adhering to a film-fixing plane around the inlet for insulating material with an adhesion higher than those of the other stacked films may be applied to the flexible film to ensure tight contacts among the films. A laminated film which has a waterproof film in at least one layer and another film with a greater resistance against the heat stress compared to that of the waterproof film in at least another layer may also be used as the flexible film.
The fifth feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; an insulating material formed on the semiconductor chip; and a foam provided on the insulating material.
According to the fifth feature of the present invention, as the foam which is filled on the insulating material can serve as a buffer against the thermal expansion and shrinkage of the insulating material such as the silicone gel, the heat stress applied to the package for the semiconductor power device may be relaxed.
Similar to the first to third features, it is possible to further comprises a container arranged on the bottom plate. Then the insulating material can be filled within the container up to a middle height; and the foam is provided on the insulating material.
The sixth feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; a container arranged on the bottom plate, surrounding the insulating substrate; a jog portion formed on a side of an upper aperture of the container; a sealing material provided in the aperture, having another jog portion corresponding to the jog portion of the aperture; and an insulating material formed on the semiconductor chip to fill the container.
According to the sixth feature of the present invention, presence of the jog portion formed on the upper part of the container may elongate a distance at an adhered interface between the container and the sealing material. The adhered interface may substantially reduce an amount of moisture that penetrates into the container through the interface, because the amount is inversely proportional to the second power of the distance.
The seventh feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate, for mounting semiconductor chip on the conductive film; a container arranged on the bottom plate, surrounding the insulating substrate; an insulating material formed on the semiconductor chip to fill the container; an upper lid arranged at an upper portion of the container; an external terminal supported through the upper lid and connected electrically with the semiconductor chip; and an insulating sheath for covering an outer surface of the external terminal.
According to the seventh feature of the present invention, even in a structure where the external terminal passes through a material with a relatively lower dielectric breakdown voltage such as the foam, the dielectric breakdown voltage can be increased. The insulating sheath which covers the external terminal passes through the upper lid and the insulating material. The insulating sheath may, for example, be made of resin. Therefore, if the dielectric constant of the sheath is lower than those of the upper lid and the insulating material, an electric field near the external terminal becomes greater than that when the external terminal is not covered with the insulating sheath. Thus, a material which has a dielectric constant higher than that of the upper lid is required as the insulating sheath for covering. It is also required, of course, that the dielectric constant of the insulating sheath for covering is higher than that of the insulating material. Thus, the dielectric breakdown voltage between the external terminals can be increased and the generation of the dielectric breakdown can be prevented.
The eighth feature of the present invention lies in a package for a semiconductor power device which comprises at least: a conductive bottom plate serving as a heat sink; an insulating substrate mounted on the bottom plate; a conductive film formed on the insulating substrate selectively to expose a peripheral region of the insulating substrate and a semiconductor chip mounted on the conductive film; a thermoplastic insulating resin formed on the semiconductor chip; and an external terminal arranged through the thermoplastic insulating resin and connected electrically with the semiconductor chip.
According to the eighth feature of the present invention, as the entire semiconductor chip is covered with the thermoplastic resin that has a high waterproof property, the degradation of the semiconductor power device due to the absorption of humidity can be prevented, and advanced electric properties can be obtained. Further, an inner structure can be simplified compared to the conventional one, the package for the semiconductor power device can be miniaturized, and the steps required to assemble the semiconductor power device can be reduced.
In the eighth feature of the present invention, the thermoplastic insulating resin can seal the insulating substrate, the semiconductor chip and at least a part of the bottom plate integrally. However, it is also possible to implement a configuration, in which the thermoplastic resin is molded only on the insulating substrate so as not to contact with the bottom plate. By molding the thermoplastic resin only on the insulating substrate without contacting to the bottom plate, a thickness of molding and an amount of the thermoplastic resin used can be reduced. Further, as the molding can be easily performed and the assembling time can be shortened, the problems during molding such as the deformation of the components which are employed to construct the device can be reduced, and the durability and reliability of the package for the semiconductor power device can be improved.
The ninth feature of the present invention lies in a method for assembling a package for a semiconductor power device which comprises at least the steps of: (a) bonding an insulating substrate on a conductive bottom plate; (b) mounting a semiconductor chip on the insulating substrate; (c) connecting electrically the semiconductor chip with an external terminal; (d) injection-molding an insulating resin on the bottom plate to form an outer case consisting of the insulating resin for surrounding the insulating substrate and the semiconductor chip; and (e) filling a insulating resin within the outer case.
According to the ninth feature of the present invention, the outer case is produced by injecting the insulating resin onto the metallic base, so the outer case and the metallic base are firmly integrated. In addition, the adhesion between the outer case and the metallic base is improved, and the moisture penetration into the device can be prevented. The insulating resin may be a thermoplastic insulating resin or another resin.
A polyphenylene sulfide may be used as the thermoplastic resin in the eighth and ninth features. Catalysts, additives and inorganic fillers may also be employed suitably to adjust the adhesion and to relax the inner stress with using the polyphenylene sulfide as a main constituent. Preferable examples of the inorganic fillers may include a fused silica, a powdered quartz, a powdered glass, a short glass fiber, an alumina (Al2O3) and an aluminum nitride (AlN). In the case where the thermoplastic resin has a two-layer structure, a sandwich-molding may be applied to mold the resin. A vacuum pressure impregnation treatment may also be employed preferably for impregnating the insulating resin into the insulating substrate in advance to improve an insulating property of the insulating substrate by repairing the inner defects of the insulating substrate. A laminated structure that consists of plural insulating substrate materials may be applied to the insulating substrate to improve the insulating property of the insulating substrate.
The xe2x80x9csemiconductor power devicesxe2x80x9d in the first through ninth features may include various power devices such as the IGBT, the Power MOSFET, the Power BJT, a Power Static Induction Transistor (SIT), a Thyristor, the GTO Thyristor and a Static Induction (SI) Thyristor. The xe2x80x9csemiconductor chipsxe2x80x9d means such semiconductor chips that have at least one of the semiconductor power devices mounted thereon. These semiconductor chips may also include a chip which has a circuit for controlling the semiconductor power device and a protection circuit mounted thereon. A power IC that integrates the control circuit on the same monolithic substrate where the semiconductor power device is formed may also be employed. At least one of these xe2x80x9csemiconductor chipsxe2x80x9d may be disposed on the conductive film. The package for semiconductor power device according to the present invention can include various electronic components, for example, resistors, capacitors, inductors, circuit wires and leads such as bonding wires.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.